1. Field of the Invention
This invention relates to improvements in fire-rated suspended ceiling constructions in which a metallic grid system is suspended from primary ceiling members and is used to support acoustical tiles or other ceiling boards or panels.
2. Description of the Prior Art
In general, suspended ceilings employ acoustical or other boards that are fire resistant or fire retardant. With the constructions that have been proposed in the prior art, however, the ability of such ceilings to provide protection against fire from below has been limited by the metallic main beams or main runners and the cross tees or cross runners that form the grid by which the boards are supported. The main beams and cross tees are generally made of metal having appreciable positive temperature coefficients of expansion and consequently tend to expand and buckle from the heat generated by fires. As a result of such expansion and buckling, boards are displaced from their position of support by the main beams and cross tees, thereby exposing the area above the suspended ceiling to the fire from below.
Various proposals have been made in the prior art for modifying suspended ceiling grids for accommodating the stress of main beam and cross tee expansion so as to maintain the proper support of the ceiling boards even during a condition of excessive heat as would be caused by a fire. The prior art proposals to accommodate, and hence, avoid the adverse effects of undue stress in the main beam and cross tee grid structure of a suspended ceiling have generally involved an attempt to control the expansion such as by weakening the main beams of a grid by slitting or cutting away portions or by providing an additional reinforcing and splicing plate or member at the coupling of the main beams. Suspended ceiling grid structures utilizing one or another of these techniques are shown in U.S. Pat. Nos. 3,175,655, 3,189,139, 3,457,688, 3,807,111, and 3,890,760. However, the weakening of the main beams detracts substantially from the rigidity of the grid and its capacity to support the ceiling boards without appreciable sagging. The use of reinforcing splice plates adds undesirably both to inventory and installation problems as well as to cost of materials. The use of heavier gauge metal to strengthen the main beams and cross tees has also been proposed. Such a solution to the problem, however, is impracticable as prohibitively expensive.